1. Field of the Invention
The present invention relates to a technique of annealing, for instance, a semiconductor material by illuminating it with laser light. The invention generally relates to techniques of processing or modifying an object in various manners by illuminating it with laser light.
The invention also relates to a laser annealing apparatus and method for annealing a semiconductor material by using a linear laser beam.
The invention is particularly effective when used, for instance, in a process of converting an amorphous silicon film into a crystalline silicon film, a process of improving the crystallinity of a crystalline silicon film, and a process of repairing lattice defects that have been generated by implanting an impurity into a crystalline silicon film to, for instance, render it conductive all of which processes are performed by laser annealing.
2. Description of the Related Art
In recent years, various studies have been made extensively to reduce the temperature of semiconductor device manufacturing processes. The major reason for this tendency is the need of forming semiconductor devices on an insulative substrate, such as a glass substrate, which is inexpensive and highly workable. Stated more specifically, this is due to the need of forming thin-film transistors of several hundred by several hundred or more on a glass substrate in producing an active matrix liquid crystal display device. Other needs such as the needs of forming finer devices and multilayered devices have also prompted the studies mentioned above.
In semiconductor manufacturing processes, it is sometimes necessary to crystallize an amorphous semiconductor material or amorphous components contained in a semiconductor material, recover the crystallinity of a semiconductor material which was originally crystalline but has been lowered in the degree of crystallinity due to ion irradiation for impurity implantation, or improve the degree of crystallinity of an already crystalline semiconductor material. Conventionally, thermal annealing is used for these purposes. Where the semiconductor material is silicon, crystallization of amorphous silicon, recovering or improvement of crystallinity, etc. are attained by performing annealing at 600 to 1,100° C. for 0.1 to 48 hours or more.
In general, the above-mentioned thermal annealing may be performed in a shorter processing time when the temperature is higher. However, it has almost no effect when the temperature is 500° C. or less. Therefore, from the viewpoint of decreasing the temperature of a process, it is necessary to replace a step that conventionally uses thermal annealing with some other means.
In particular, where a glass substrate is used, it is required that the thermal annealing temperature be 700° C. or less, and that the heating time be as short as possible. The latter requirement is due to the fact that a long heat treatment may deform the glass substrate. In a liquid crystal display device, a liquid crystal is held between a pair of glass substrates having a gap of several micrometers. Therefore, deformation of the glass substrates greatly affects display performance of the liquid crystal display device.
Various types of annealing technique using laser light illumination are known as processes for replacing the thermal annealing. Laser light can impart high energy that is equivalent to the energy obtained by the thermal annealing only to a desired portion; it is therefore not necessary to expose the entire substrate to a high-temperature atmosphere.
Stated in general, there have been proposed the following two laser light illumination methods:
In the first method, a CW laser such as an argon ion laser is used and a spot-like beam is applied to a semiconductor material. A semiconductor material is crystallized such that it is melted and then solidified gradually due to a sloped energy profile of a beam and its movement.
In the second method, a pulsed oscillation laser such as an excimer laser is used. A semiconductor material is crystallized such that it is melted instantaneously by application of a high-energy laser pulse and then solidified.
The first method has a problem of long processing time, because the maximum energy of a CW laser is insufficient and therefore the beam spot size is at most several square millimeters. In contrast, the second method can provide high mass-productivity, because the maximum energy of a laser is very high and therefore the beam spot size can be made several square centimeters or larger.
However, in the second method, to process a single, large-area substrate with an ordinary square or rectangular beam, the beam needs to be moved vertically and horizontally, which inconvenience still remains to be solved from the viewpoint of mass-productivity.
This aspect can be greatly improved by deforming a laser beam into a linear shape and moving the linear beam approximately perpendicularly to its longitudinal direction to effect scanning. The term “scanning” as used in this specification means illuminating an object while moving a linear laser beam step by step with an overlap in the beam width direction, that is, approximately perpendicularly to the longitudinal direction of the beam.
The problem remaining unsolved is insufficient uniformity of laser light illumination effects. The following measures have been taken to improve the uniformity. A first measure is to make the beam profile as close to a rectangular one as possible by causing a laser beam to pass through a slit, to thereby reduce an energy variation within a linear beam.
FIGS. 4A and 4B show an energy profile of a laser beam; FIG. 4A shows an example of a rectangular energy profile. The term “rectangular” as used in this specification means a relationship L2, L3≦0.2L1 where L1 to L3 are defined in FIG. 4B.
In using the above technique, it has been reported that the uniformity can further be improved by performing preliminary illumination with weaker pulse laser light before illumination (hereinafter called “main illumination”) with stronger pulse laser light.
This measure is so effective that the characteristics of resulting semiconductor devices can be improved very much. This is because the two-step laser light illumination with different illumination energy levels allows a semiconductor film to be crystallized step by step, thereby reducing the seriousness of such problems as a non-uniform distribution of crystallinity, formation of crystal grains, and concentration of stress, which problems result from abrupt phase changes.
The stepped illumination can be made more effective by increasing the number of illumination steps.
Thus, the above two kinds of measure can greatly improve the uniformity of the laser light illumination effects.
However, with the above two-step illumination method, the laser processing time is doubled, that is, the throughput is reduced.
Further, the equipment for the two-step illumination method is more complex than that for the single step illumination method, thus causing a cost increase.
In addition, although the above measures have much improved the uniformity of the laser light illumination effects, the degree of improvement is still insufficient.
To transform a square or rectangular light beam into a linear beam, a specialized optical system is needed.
FIG. 14 shows an example of an optical system of a conventional laser annealing apparatus.
The optical system of FIG. 14 is composed of the following components. An excimer laser beam generating means A′ generates an excimer laser beam. Beam expanders B′ and C′ expand the excimer laser beam. A vertical expansion fly-eye lens D′ and a horizontal expansion fly-eye lens D2′ expand the laser beam in a sectional manner. A first cylindrical lens E′ converges the laser beam into a line shape. A second cylindrical lens F′ improves the uniformity of the linear laser beam in its longitudinal direction. A stage I′ is moved in direction J′ indicated by an arrow in FIG. 14 in a state that an illumination object, a substrate bearing an illumination object, or the like is placed thereon.
In FIG. 14, a path-folding mirror G′ and a cylindrical lens H′ serve to apply the laser beam to an object on the stage I′. In certain types of configuration, the beam expanders B′ and C′ are omitted.
A uniform linear laser beam can be obtained by the above optical system. However, in this conventional optical system, the use of two fly-eye lenses for sectionally expanding a laser beam, that is, the fly-eye lens D′ for vertical expansion and the fly-eye lens D2′ for horizontal expansion, lowers the transmittance of the entire fly-eye lens system, resulting in a low laser beam energy efficiency. As a result, in laser annealing, the amount of energy applied to an illumination object may be lowered, possibly making the annealing insufficient.
To prevent this problem, the output of the laser light source needs to be increased. But this increases the load on the laser light source, so that the life of the entire apparatus may be shortened.